Tip: If you make a change to your ROM that you regret, just go to File > Restore > Restore ROM to Previous State in the menu. From there, you can select a restore point and revert your ROM to a previous state.
Last X/Y value of interactive layer 2 where VRAM upload of Map16 tiles was performed when scrolling right/down.
The low 4 bits are forced to zero (AND #$FFF0) in order to get scroll values on a 16 pixel boundary.
It is used to determine if a VRAM update is necessary during scrolling.